skiplistrep.cc

//  Copyright (c) 2013, Facebook, Inc.  All rights reserved.
//  This source code is licensed under the BSD-style license found in the
//  LICENSE file in the root directory of this source tree. An additional grant
//  of patent rights can be found in the PATENTS file in the same directory.
//
#include "rocksdb/memtablerep.h"
#include "db/memtable.h"
#include "db/skiplist.h"
#include "util/arena.h"

namespace rocksdb {
namespace {
class SkipListRep : public MemTableRep {
  SkipList<const char*, const MemTableRep::KeyComparator&> skip_list_;
  const MemTableRep::KeyComparator& cmp_;
  const SliceTransform* transform_;
  const size_t lookahead_;

  friend class LookaheadIterator;
public:
  explicit SkipListRep(const MemTableRep::KeyComparator& compare,
                       MemTableAllocator* allocator,
                       const SliceTransform* transform, const size_t lookahead)
    : MemTableRep(allocator), skip_list_(compare, allocator), cmp_(compare),
      transform_(transform), lookahead_(lookahead) {
  }

  // Insert key into the list.
  // REQUIRES: nothing that compares equal to key is currently in the list.
  virtual void Insert(KeyHandle handle) override {
    skip_list_.Insert(static_cast<char*>(handle));
  }

  // Returns true iff an entry that compares equal to key is in the list.
  virtual bool Contains(const char* key) const override {
    return skip_list_.Contains(key);
  }

  virtual size_t ApproximateMemoryUsage() override {
    // All memory is allocated through allocator; nothing to report here
    return 0;
  }

  virtual void Get(const LookupKey& k, void* callback_args,
                   bool (*callback_func)(void* arg,
                                         const char* entry)) override {
    SkipListRep::Iterator iter(&skip_list_);
    Slice dummy_slice;
    for (iter.Seek(dummy_slice, k.memtable_key().data());
         iter.Valid() && callback_func(callback_args, iter.key());
         iter.Next()) {
    }
  }

  uint64_t ApproximateNumEntries(const Slice& start_ikey,
                                 const Slice& end_ikey) override {
    std::string tmp;
    uint64_t start_count =
        skip_list_.EstimateCount(EncodeKey(&tmp, start_ikey));
    uint64_t end_count = skip_list_.EstimateCount(EncodeKey(&tmp, end_ikey));
    return (end_count >= start_count) ? (end_count - start_count) : 0;
  }

  virtual ~SkipListRep() override { }

  // Iteration over the contents of a skip list
  class Iterator : public MemTableRep::Iterator {
    SkipList<const char*, const MemTableRep::KeyComparator&>::Iterator iter_;
   public:
    // Initialize an iterator over the specified list.
    // The returned iterator is not valid.
    explicit Iterator(
      const SkipList<const char*, const MemTableRep::KeyComparator&>* list
    ) : iter_(list) { }

    virtual ~Iterator() override { }

    // Returns true iff the iterator is positioned at a valid node.
    virtual bool Valid() const override {
      return iter_.Valid();
    }

    // Returns the key at the current position.
    // REQUIRES: Valid()
    virtual const char* key() const override {
      return iter_.key();
    }

    // Advances to the next position.
    // REQUIRES: Valid()
    virtual void Next() override {
      iter_.Next();
    }

    // Advances to the previous position.
    // REQUIRES: Valid()
    virtual void Prev() override {
      iter_.Prev();
    }

    // Advance to the first entry with a key >= target
    virtual void Seek(const Slice& user_key, const char* memtable_key)
        override {
      if (memtable_key != nullptr) {
        iter_.Seek(memtable_key);
      } else {
        iter_.Seek(EncodeKey(&tmp_, user_key));
      }
    }

    // Position at the first entry in list.
    // Final state of iterator is Valid() iff list is not empty.
    virtual void SeekToFirst() override {
      iter_.SeekToFirst();
    }

    // Position at the last entry in list.
    // Final state of iterator is Valid() iff list is not empty.
    virtual void SeekToLast() override {
      iter_.SeekToLast();
    }
   protected:
    std::string tmp_;       // For passing to EncodeKey
  };

  // Iterator over the contents of a skip list which also keeps track of the
  // previously visited node. In Seek(), it examines a few nodes after it
  // first, falling back to O(log n) search from the head of the list only if
  // the target key hasn't been found.
  class LookaheadIterator : public MemTableRep::Iterator {
   public:
    explicit LookaheadIterator(const SkipListRep& rep) :
        rep_(rep), iter_(&rep_.skip_list_), prev_(iter_) {}

    virtual ~LookaheadIterator() override {}

    virtual bool Valid() const override {
      return iter_.Valid();
    }

    virtual const char *key() const override {
      assert(Valid());
      return iter_.key();
    }

    virtual void Next() override {
      assert(Valid());

      bool advance_prev = true;
      if (prev_.Valid()) {
        auto k1 = rep_.UserKey(prev_.key());
        auto k2 = rep_.UserKey(iter_.key());

        if (k1.compare(k2) == 0) {
          // same user key, don't move prev_
          advance_prev = false;
        } else if (rep_.transform_) {
          // only advance prev_ if it has the same prefix as iter_
          auto t1 = rep_.transform_->Transform(k1);
          auto t2 = rep_.transform_->Transform(k2);
          advance_prev = t1.compare(t2) == 0;
        }
      }

      if (advance_prev) {
        prev_ = iter_;
      }
      iter_.Next();
    }

    virtual void Prev() override {
      assert(Valid());
      iter_.Prev();
      prev_ = iter_;
    }

    virtual void Seek(const Slice& internal_key, const char *memtable_key)
        override {
      const char *encoded_key =
        (memtable_key != nullptr) ?
            memtable_key : EncodeKey(&tmp_, internal_key);

      if (prev_.Valid() && rep_.cmp_(encoded_key, prev_.key()) >= 0) {
        // prev_.key() is smaller or equal to our target key; do a quick
        // linear search (at most lookahead_ steps) starting from prev_
        iter_ = prev_;

        size_t cur = 0;
        while (cur++ <= rep_.lookahead_ && iter_.Valid()) {
          if (rep_.cmp_(encoded_key, iter_.key()) <= 0) {
            return;
          }
          Next();
        }
      }

      iter_.Seek(encoded_key);
      prev_ = iter_;
    }

    virtual void SeekToFirst() override {
      iter_.SeekToFirst();
      prev_ = iter_;
    }

    virtual void SeekToLast() override {
      iter_.SeekToLast();
      prev_ = iter_;
    }

   protected:
    std::string tmp_;       // For passing to EncodeKey

   private:
    const SkipListRep& rep_;
    SkipList<const char*, const MemTableRep::KeyComparator&>::Iterator iter_;
    SkipList<const char*, const MemTableRep::KeyComparator&>::Iterator prev_;
  };

  virtual MemTableRep::Iterator* GetIterator(Arena* arena = nullptr) override {
    if (lookahead_ > 0) {
      void *mem =
        arena ? arena->AllocateAligned(sizeof(SkipListRep::LookaheadIterator))
              : operator new(sizeof(SkipListRep::LookaheadIterator));
      return new (mem) SkipListRep::LookaheadIterator(*this);
    } else {
      void *mem =
        arena ? arena->AllocateAligned(sizeof(SkipListRep::Iterator))
              : operator new(sizeof(SkipListRep::Iterator));
      return new (mem) SkipListRep::Iterator(&skip_list_);
    }
  }
};
}

MemTableRep* SkipListFactory::CreateMemTableRep(
    const MemTableRep::KeyComparator& compare, MemTableAllocator* allocator,
    const SliceTransform* transform, Logger* logger) {
  return new SkipListRep(compare, allocator, transform, lookahead_);
}

} // namespace rocksdb